The Callback and TypedArray are the only JavaScript types supported by
the library, thus they are special-cased in a type switch of ValueOf.

Instead, a Ref interface is defined to allow external wrapper types
to be handled properly by ValueOf.

Change-Id: I03240ba7ec46979336b88389a70b7bcac37fc715
GitHub-Last-Rev: c8cf08d8ccfaab2af98df9eec8bc7b60dbce2c64
GitHub-Pull-Request: golang/go#28181
Reviewed-on: https://go-review.googlesource.com/c/141644
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Richard Musiol <neelance@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>